Water-soluble cellulose derivatives, such as water-soluble cellulose ethers, are obtained by treating cellulose having crystalline and amorphous moieties within the molecule with a derivatizing agent, such as an etherifying agent, for converting the crystalline moieties to an amorphous state whereby the cellulose derivative is made water soluble. It is acknowledged that the crystallinity of cellulose is largely ascribed to hydrogen bonds between hydroxyl groups within the molecule which develop on the skeleton structure of cellulose molecule. Cellulose is water insoluble because the hydrogen bonds are strong enough to prevent hydration with water molecules in water. In the preparation of cellulose derivatives, such as cellulose ethers, cellulose is treated with an alkaline aqueous solution such as NaOH to convert to alkali cellulose for disrupting crystallinity, and then reacted with a derivatizing agent, such as an etherifying agent, to substitute the derivatizing agent for hydroxyl groups on the cellulose to form a cellulose derivative. Ideally the cellulose derivative can be fully dissolved in water to form a transparent aqueous solution. However, since crystallinity has not completely disappeared in the alkali cellulose, commercially available water-soluble cellulose derivatives can partially contain water-insoluble portions, which are difficult and time-consuming to remove from the water-soluble cellulose derivative, for example by filtration.
However, the residual amount of water-insoluble fibers is undesirable in many end-use applications. For example, water-soluble cellulose derivatives are used as a thickener for clear shampoo/rinses, hair conditioners, eye drops, contact lens cleaners, formulations for dipping solutions for capsule manufacture, formulations for clear tablet coatings, and the like. In these applications, products are desired to be clear. A substantial amount of water-insoluble fibers in water-based solutions of water-soluble cellulose derivatives will lead to deficient transparency. Moreover, certain cellulose ethers, such as METHOCEL™ F4M cellulose ether and METHOCEL™ A4M cellulose ether, commercially available from The Dow Chemical Company, are used as binders/plasticizers in ceramic-forming materials, such as thin-walled honeycomb structures for use as a carrier for a catalyst or a catalyst filter for exhaust gas treatment. A substantial amount of water-insoluble fibers in water-soluble cellulose ethers used as binders/plasticizers in thin-walled honeycomb structures lead to reduced stability and defects, such as pinholes, in the thin-walled honeycomb structures. Japanese Patent Application Publication JP1111770A suggests incorporating a cellulose derivative leaving ≦1,000 units of undissolved fibers having 8 to 200 μm diameter in 2 cm3 of its 0.1 wt. % aqueous solution as the molding binder for ceramic material. JP1111770A discloses that when using this cellulose derivative a molded product is obtained wherein pores are not formed after sintering.
European Patent application EP 1 375 523 discloses that aqueous solutions of cellulose ethers are filtered to remove those portions which have not been dissolved on the molecular level, prior to use. However, EP 1 375 523 also discusses that in the filtering process the filter is frequently clogged, which obstructs the industrial manufacture. To solve this problem, EP 1 375 523 suggests a method of selecting as a water-soluble cellulose ether whose number of undissolved fibers having a size of 16 to 200 μm in a 0.1 wt % aqueous solution at 25° C. is up to 350 fibers/2 ml. An aqueous solution of such selected water-soluble cellulose ether is said to cause minimized clogging to a metal mesh having an opening of less than 63 μm.
However, there is a strong need not just to select cellulose ethers with a low level of undissolved fibers but to find a process for reducing the amount of water-insoluble fibers in water-soluble cellulose derivatives.